I'll take a crack at this. Maybe someone can help me out if I make a mistake.
We only have two ears and the piano only has two outs, so it would seem that you can only get as much of a soundstage as can be defined by the differences between the sound coming in one ear and the other. In actuality this is not true because we hear not only with our ears but with the skull behind our ears and other parts of our head and somehow our body uses this information to construct a more vivid soundstage. Headphones not only send sound to our eardrums but if they are circumaural send some sound behind our ears, etc. and it's not just the same sound as goes into our eardrum. This information is used by our brain to create what feels like a 360 musical experience.
You can't get the same experience out of two speakers easily in my experience. That's why we have surround sound in some cases (not generally in digital pianos, though). If you read headphone forums they are always talking about "soundstage." This is what they are referring to.
To be fair, hi fi people use the term as well, even when talking only about stereo speakers. I guess the same effect is available but not to the same degree perhaps.
Since studio monitors are super directional, I definitely don't get a 360 soundstage when I personally listen to them.
Maybe it's not the same with dipole-style speakers (magnepan and the like).
Edited by gvfarns (01/02/12 01:48 PM)